One exemplary configuration of known bias circuits used for amplifiers, in particular high frequency amplifiers, includes a separate current-generating transistor in addition to an amplifying transistor, and decides a bias current of the amplifying transistor on the basis of a difference in the current flowing in these two transistors. In this configuration, it is possible to decide the current that will flow to the amplifying transistor by setting beforehand the current to flow to the current-generating transistor.
In such configuration, if the current of the current-generating transistor decreases due to certain changes in the surround environment (e.g., an increase in the ambient temperature), the current of the amplifying transistor also decreases. In other words, there is a problem that a gain of the amplifier drops as the current of the current-generating transistor drops due to the variations in the surrounding environment.